[stylesheets] Updated Stylesheets to look good on all platforms (Closes #276)

GUI/Application/ApplicationSettings.cpp

GUI/Application/ApplicationSettings.h

GUI/Support/IO/AbstractDataLoaderResultModel.cpp

GUI/View/Common/DocksController.cpp

GUI/View/Common/StyledToolbar.cpp

GUI/View/Common/StyledToolbar.h

GUI/View/Fit/MinimizerSettingsWidget.cpp

GUI/View/Import/RealDataSelectorWidget.cpp

GUI/View/Import/RealDataTreeModel.cpp

GUI/View/Instrument/AxisPropertyEditor.cpp

GUI/View/Instrument/DistributionEditor.cpp

GUI/View/Instrument/FootprintCorrectionEditor.cpp

GUI/View/Instrument/InclinationAnglesEditor.cpp

GUI/View/Instrument/InstrumentLibraryEditor.cpp

GUI/View/Instrument/InstrumentView.cpp

GUI/View/Instrument/InstrumentView.h

GUI/View/Instrument/InstrumentsTreeModel.cpp

GUI/View/Instrument/RectangularDetectorEditor.cpp

GUI/View/Instrument/ResolutionFunctionEditor.cpp

GUI/View/Main/ActionManager.cpp